1. Field of the Invention
The present invention is in the field of high efficiency amplifiers. The present invention is also in the field of actuator drivers and controllers. The present invention further relates to motor controllers. The present invention further relates to Hard Disk Drives and optical data storage devices. The present invention further relates to methods and circuits for controlling a voice coil motor for positioning the read/write head of a hard disk drive. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into a larger integrated circuit.
The invention also falls within the field of integrated circuits to drive a loudspeaker, an actuator or a motor.
2. Brief Description of Related Art
The physical kinetic parameters of a motor such as velocity and acceleration are directly linked to its torque which, in its turn, is directly dependent on the current applied to the motor itself. That is why most motors are driven in current by means of a control loop that senses the current in the motor and regulates it according to a desired value.
In several fields the accurate control of position, velocity and acceleration of a motor is critical to the overall performance of the system. Some of these fields are: the hard disk drive applications, the optical data storage motor positioning applications, the digital still camera applications to control focus, zoom and other dedicated motors, the printer applications, the robotics and others.
The position of the read/write head of a disk drive is typically controlled by a linear motor, often referred to as the Voice Coil Motor (VCM) that moves a mechanical arm over the disk surface. The VCM 4, as shown in FIG. 1, is represented as an inductor L1 in series to a resistor R1 to indicate the main electrical parameters of the motor. This representation does not include the Back Electromotive Force (BEMF) that is generally represented as a series voltage generator whose value is dependent on the velocity of the motor. The VCM is driven in response to a control loop, known as the servo loop, whose main algorithm is implemented typically within a microprocessor or similar digital processor, and is typically driven in at least three different modes.
A “seek” mode causes the read/write head to move from one track on the disk to a potentially unrelated track, which may require a significant motion. In this mode, the control system typically attempts to control the velocity of the mechanism. In “track follow” mode, the read/write head is relatively stationary, and the control system works to control its precise position to be directly above the appropriate track. In a third mode, the head is driven onto or off of the disk surface to a “park” position, typically using a mechanical ramp to pull the head above the surface of the disk.
As shown in FIG. 1, the VCM control system comprises a serial port 1 that communicates with the microprocessor that contains the main servo algorithm and that drives, with digital signals, a digital to analog converter (DAC) 2. This DAC 2 typically drives a VCM actuator 11 in its various forms and implementations. The VCM actuator 11 commands the current into the VCM 4 which defines its arm's velocity and position on the disk surface.
In addition to the servo loop there is, typically, an inner analog current control loop that drives the VCM as shown in more details in FIG. 1. The serial port 1 drives a Digital to Analog Converter (DAC) 2 which, in its turn, commands the current through the inner current control loop. In this case the VCM actuator block 11 comprises the inner analog current control loop to regulate the current into the VCM 4.
In order to obtain optimal control, the overall servo loop commands a particular current to be driven into the VCM, and an inner analog control loop regulates the current. Practical circuit implementation considerations require that the VCM be driven with conventional amplifiers which impose a voltage across the VCM. The local analog control loop senses the current in the VCM, compares it to the commanded current, and adjusts the drive voltage to maintain the desired current.
The inner analog control loop is driven by a DAC 2 creating an analog representation of the digitally commanded current, and a Current Sense Amplifier (CSA) 5 generates a signal representing the instantaneous value of the VCM current. These two signals are summed at the input of the error amplifier 6 via resistors R2 and R3 respectively, and this sum is the error in the value of the current. The voltage reference 3 sets the common mode voltage at the load.
The error amplifier 6 is conventionally an integrator, with arbitrarily high gain at DC but with gain falling with frequency to maintain the stability of the loop at higher frequencies. This stage might also implement additional frequency/phase shaping for stability. Such frequency response shaping is controlled by C1, C2 and R4, as is well known in the art. The output of error amplifier 6 feeds the pseudo class AB stages 9 and 10 which are typically constituted of two anti-phase linear amplifiers 7 and 8, coupled to a “full bridge” capable of applying the full supply voltage across the load in either polarity. The linear amplifiers 7 and 8, even if not depicted in details in FIG. 1 for simplicity, are typically configured as closed loop amplifiers with a constant gain defined by feedback resistors. In series with the VCM 4 there is a power resistor R5 used to sense current. The voltage across this current sense resistor R5 is used as the differential input to the current sense amplifier 5.
Within this loop, the error amplifier is a large bandwidth standard operational amplifier. The DC errors can be initialized out of the loop with software, during the so called “calibration phase” and the AC requirements are generally met with conventional design techniques. The VCM power amplifiers 7 and 8 are similarly very conventional in design. Typical Class AB stages are implemented with complementary components biased with a stand-by current in order to achieve very low zero-cross distortion.
Zero-cross distortion is an important parameter to measure the ability of the driver to exhibit zero current in the motor when zero current is desired. The so-called “jumps” or “dead-bands” in the transfer function of the amplifier are highly undesirable and typically minimized by the use of class AB stages. When the stages are biased in a similar manner using non-complementary components, as is often the case for the integrated motor driver circuits, they are generally known as pseudo-class AB amplifiers.
The overall analog control system, including DAC, current sense amplifier, error amplifier and power amplifiers is typically implemented on a single chip, usually along with the control and power stage for the disk drive spindle motor actuator and any other analog/power functions required in the system. The resultant chip's efficiency is determined by the efficiency of all the subsystems, but in particular the product of current and voltage for the output transistors in the diverse conditions of the motor drive is the main contributor to the power dissipation in the chip.
In the case of the VCM, depending on the modes of operation, the drive may be more or less efficient. Typically in “track follow” mode the current is not very significant, but the voltage might be (depending on the voltage common mode of the output stage). In “track follow” the current is mainly due to the fact that the arm of the VCM has to overcome the spring force of the flexible connector that carries the conductors for the pre-amplifier located on the tip of the arm, therefore the current is depending on the location of the arm, whether it is closer to the center or to the outer track of the disk. In “seek” mode the current is quite high, but the voltage between drain and source of the power transistors of the bridge output stage is generally not very high, since they are typically operating in the triode region.
Generally the maximum power dissipation occurs during the transition between these two main modes of operation and more specifically during the acceleration and deceleration of the VCM arm, when the product of current and voltage applied to the power stages in the chip is significant also for effect of the back electromotive force. In fact, in order to optimize the mechanical response of the motor and minimize the seek time a pseudo-sinusoidal profile is given to the current in the motor.
Nowadays several efforts are increasingly made to improve the overall efficiency of the motor drive especially for the case of battery operated disk drive or more generally motor drives. Class AB amplifiers, although featuring low overall distortion, are constantly biased at a not negligible stand-by current that, if combined with high voltage drop, constitutes a considerable power loss.
The utilization of PWM switching approaches, such as driving a motor in class-D or with more traditional PWM control loops, introduces high frequency switching noise that can interfere with the operation of the device. Furthermore the magnetic losses in the motor due to high frequency switching at its terminals may negatively impact the overall system efficiency. In the case of the VCM, the Hard Disk Drive manufacturers have been reluctant to employ these approaches despite of their recognized advantages in terms of reduced power dissipation.
The proliferating use of miniature precision motor drive in battery operated devices is posing two formidable related problems: a) extending the time between charges and b) being able to dissipate the necessary power in order to keep the device temperature within reasonable ranges.
The solution to both these problems is to find accurate and more efficient means to drive the motors. In particular for brushless DC motors used in Hard Disk Drive and data storage devices, as well as in digital still cameras, the efficiency is becoming a very critical aspect of their overall performance. The typical case could be the VCM of the Hard Disk Drive. In this case, even for desktop applications, that are not battery operated, the efficiency is increasingly an important factor due to the fact that higher processor speeds, within the personal computer case, tend to raise the temperature rapidly.
The use of PWM motor drives that apply an average voltage at the terminals of the motor driving fully on or fully off the power transistors at frequencies in the range of 100 KHz to a few MHz is very well known to those skilled in the art. However these schemes have several disadvantages such as higher harmonic distortion, higher complexity, high magnetic losses in the motor and most importantly the Electro Magnetic Interference (EMI) effects generated by the fast voltage rising and falling edges at the motor terminals.
In particular, the EMI has limited the use of switching drive methods in cases like the Voice Coil Motor drive especially in “track follow”. The proposed invention makes use of output stages (typically in full bridge configuration) that get current from a modulated supply voltages controlled by a feedback control system in order to operate with the minimum bridge common mode voltage (CMV) so as to guarantee very low distortion even in presence of varying BEMF.
The output stage power supply is modulated by a switching converter to obtain high efficiency. This configuration is similar to the not widely known class H stage amplifiers. The switching converter could be a step up or a step down converter and in particular, in order to obtain good efficiency in all the load conditions, it is advantageous to apply a step down conversion in a manner analogous to the operation of a buck converter.
The first official document describing a rudimentary approach similar to what is nowadays known as class H output stage, is Jensen (U.S. Pat. No. 3,426,290). Jensen in 1965 described a system to modulate the power supplies of an amplifier by means of a switching regulator circuit to minimize the dissipation in the output stage.
Similarly Hamada (U.S. Pat. No. 4,054,843) in 1976 described a simple step down converter to modulate the supply of an emitter follower stage implemented with bipolar transistors depending on the input signal amplitude. Although this implementation does not provide the performances of modern class H amplifiers, it certainly characterizes the main concept.
Several other tracking power supply systems have been disclosed in Iwamatsu (U.S. Pat. No. 4,087,759) issued May 2, 1978, Carver (U.S. Pat. No. 4,218,660) issued Aug. 19, 1980, Garde (U.S. Pat. No. 4,378,530) issued Mar. 29, 1983 and Amada et al. (U.S. Pat. No. 4,409,559) issued Oct. 11, 1983. In all these examples the dual supplies are controlled by means of signals derived by the input signal to be amplified.
French (U.S. Pat. No. 5,075,634) describes a bridge output stage whose supply voltage is generated by a PWM (Pulse Width Modulation) open loop system controlled by the input signal. The main signal path to the amplifier is delayed by a delay stage to allow sufficient time to the tracking supply to react to a fast amplitude change of the signal.
Further similar approaches of amplifiers having dual modulated power supply rails based on the amplitude of the input signal are disclosed in Andersson et al. (U.S. Pat. No. 5,200,711), Williamson et al. (U.S. Pat. No. 5,396,194), Carver (U.S. Pat. No. 6,104,248) and Higashiyama et al. (U.S. Pat. No. 6,166,596).
A slightly different approach is disclosed in Johnson (U.S. Pat. No. 6,304,138). Johnson describes a bridged output stage whose supply voltage is first selected between two separate rails and then modulated by the voltage present at the output terminals of the bridge stage. This method constitutes a rather inefficient means of minimizing power dissipation because the dissipation is simply transferred to the controlling MOSFET (520 in FIG. 5) connected to the high voltage rail, operating in its linear region, when the amplifier power is not supplied by the lower voltage rail. In addition this system may cause start up problems certainly not acceptable for motor control systems.
Ricotti (U.S. Pat. No. 6,847,182) describes a method to increase the power supply voltage of a motor drive system above its BEMF (back electromotive force) by means of a boost circuit.
With the exception of Ricotti all the above-mentioned documents describe prior art class H amplifiers that are mainly used in audio applications. The present invention proposes a class H full bridge stage whose power is supplied by means of a fast power converter controlled by a feedback system. The closed loop control system guarantees that the common mode voltage at the output terminals of the bridge amplifier is regulated at an optimal voltage.
Although the proposed invention is mainly directed at precision motor drivers, it may generally be utilized for various applications (including audio) and it differentiates from all the prior art for the fact that its unique and very effective approach does not utilize the input signal to determine the amplitude of the modulated supply voltage in an open loop configuration. This represents a valid alternative to PWM schemes and class D amplifiers.
In the Hard Disk Drive (HDD) systems the density of the magnetic data recorded on the disk is increasing very rapidly and that is translated in the number of rotational tracks per inch on the disk surface. The tracks containing the magnetic data are consequently getting narrower and the burden to stay on track with limited Bit Error Rate (BER) during normal operation is shifted to the ability to control the position of the head on the disk with increasing accuracy.
It is therefore advantageous to reduce as much as possible the sources of electrical noise in the overall drive control loop so that the effective dynamic range is improved. Furthermore the switching noise at the motor terminals introduced by conventional PWM schemes may result unacceptable in the case of data storage systems that utilize magnetic media.
Accordingly, what is needed is a motor actuator that drives the motor with accuracy and high efficiency while maintaining very low cross-over distortion and without introducing the undesirable EMI effects typical of switching amplifiers (class-D) and of Pulse Width Modulation systems in general when the motor terminals are switching at high frequencies.